1. Field of the Invention
The present invention relates to the field of micro-controller devices. More specifically, the present invention relates to the protocols used in bussing and networking of micro-controller devices.
2. Description of Related Art
Certain bussing and networking protocols are based upon transmitting time dependent symbols rather than bits. One such protocol, which is used in automotive production vehicles for data-sharing and diagnostics, is the SAE J1850 Standard, published in "Class B Data Communications Network Interface," Rev. May 1994 by the SAE Vehicle Network for Multiplexing and Data Communications Standards Committee. Recently, Intel Corporation released a protocol handler to be integrated onto micro-controller devices for in-vehicle networks. These in-vehicle networks allow for better on-board diagnostics and allow sensors employed within the automobile to share data to eliminate redundant processing and sensing of information. In-vehicle networks are classified as follows according to the rate of data transmission they are capable of: Class A--Less than 10 Kilobits/second, Class B--Between 10 Kilobits/second and 125 Kilobits/second and Class C--Greater than 125 Kilobits/second. The J1850 protocol is of the Class B type of vehicle networks which are used for diagnostic and non-real-time data sharing applications (those not requiring high speed data transmission for on-the-fly vehicle control).
Micro-controllers, which are primarily digital and which employ J1850 and other such serial data transmission protocols can interface with remote transceivers to lower system cost and add flexibility. As a result, micro-controllers must interface with transceivers which convert TTL Level Digital signals to J1850 bus signals and vice versa. When the transceiver converts signals from the micro-controller, an inherent propagation delay adversely affects the timing of the J1850 symbol as detected on the J1850 bus when the micro-controller is transmitting. Likewise, when the micro-controller is receiving signals from the J1850 bus, another propagation delay due to the transceiver converting the J1850 signals to TTL signals, adversely affects the timing of symbols as detected at the input of the micro-controller.
One solution has been to hardwire delay compensation into each transceiver. However, since compensation must be achieved for the timing of specific bus protocols as well as the transceiver itself, such a solution is inadequate. Furthermore, introducing delay compensation into an analog device entails combining analog and digital circuitry in the same manufacturing process which adds to cost, size and complexity of the transceivers. Another solution is to couple delay compensation onto the micro-controller. However, to date, such solutions are based upon fixed delay compensation without regard to the transceiver specifics. This approach fails to account for the ability of the micro-controller to interface to transceivers with wide specification tolerances and with different types and models of transceivers, each with different delay characteristics.
Thus, a flexible approach is needed that can account for the differences in propagation delay from one model of transceiver to the next so that a micro-controller can be reliably interfaced with any transceiver device, and maintain accurate bus symbol timing when transmitting and receiving signals on the J1850 bus.